1. Field of the Invention
This invention relates generally to waste conversion processes, and, more particularly, to methods for abating the release into the environment of ammonia produced by anaerobic digestion.
2. Description of Related Art
Waste conversion processes such as anaerobic digestion convert protein and other nitrogenous compounds to ammonia that is discharged with the liquid and solid slurry (digestate). Typically the slurry is discharged to holding ponds or the land as a fertilizer. The solid portion of the digestate may be separated from the liquid. Typically the liquid containing ammonia is returned to an aerobic waste treatment plant or discharged to land or water bodies where adverse environmental and economic consequences can occur. The uncontrolled discharge of ammonia to the atmosphere may cause its reaction with atmospheric NOx leading to the creation of fine particulate matter (<PM2.5) that may create a significant health hazard. The ammonia eventually undergoes nitrification and denitrification, a process that generates nitrous oxide, a powerful greenhouse gas. Uncontrolled discharges of ammonia fertilize land and water creating unforeseen ecological damage. Consequently, there exists a need to control ammonia emissions from the anaerobic digestate.
The anaerobic decomposition of organic substrates also produces a biogas containing methane gas, carbon dioxide, and traces of ammonia, and hydrogen sulfide. In many cases it is desirable to produce a higher quality biogas consisting of primarily methane gas and having a high BTU content. High BTU biomethane has a significantly greater economic value if sold as biomethane than if used to produce electricity.
In the anaerobic digestion process the quantity of methane, carbon dioxide, ammonia nitrogen, and hydrogen sulfide formed is a function of the chemical composition of the substrate and the efficiency (percent conversion of organic matter) of the anaerobic digestion process. Ammonia, hydrogen sulfide, and carbon dioxide exist both as a gas (nonionized NH3, CO2, H2S) and in the ionized form (HCO3−, NH4+, HS−). The total nitrogen, carbon and sulfur are partitioned between the gas form and ionized form as a function of pH and temperature. At the mesophilic temperature and relatively neutral pH of an anaerobic reactor only a small percentage of the total ammonia is present in the gas form whereas approximately fifty percent of the total carbon and sulfide resides in the gas form. The ammonia, carbon dioxide, and hydrogen sulfide gas present in the headspace of an anaerobic reactor is a function of the solubility of the gas in solution, all in accordance with Henry's Law. Typically the volume percent, and thus partial pressure, of the digester gas is 65% methane, 34% CO2 with a small percentage being hydrogen sulfide (200-3,000 ppm) and other gases. There is very little ammonia nitrogen in the digesters headspace and thus very little ammonia nitrogen in solution in the gaseous form.
Upon exiting the anaerobic reactor, CO2 gas is discharged from the slurry to the atmosphere since the partial pressure of CO2 in the atmosphere is only 0.038% or 1/1000 the CO2 partial pressure in the digester. The loss of CO2 causes the pH of the slurry to increase resulting in the eventual shifting of the ionized ammonia (NH4+) to the unionized gas form (NH3) and the subsequent discharge of ammonia gas to the atmosphere.
Many strategies have been developed to remove and sequester ammonia nitrogen from the effluent of an anaerobic reactor. The basic strategy is to remove the ammonia from solution and form a second liquid or solid ammonium compound. Removing the ammonia from the digester effluent is normally preceded by decarbonization to remove CO2, followed by the addition of a chemical reagent, such as calcium, sodium or magnesium hydroxide to raise the solution pH and thereby shift the ionized ammonium to the unionized ammonia gas form (U.S. Pat. No. 4,104,131). Air containing a low concentration of ammonia is then used to strip the ammonia gas from solution. Steam has also been used to raise temperature, reduce the solubility of carbon dioxide, increase the pH, and strip ammonia gas from solution by reducing the pressure and thereby decreasing the partial pressure of CO2 (U.S. Pat. No. 6,521,129). High temperature (60-70° C.) reduced pressure (0.25-0.75 bar) stripping has also been proposed (U.S. Pat. No. 6,368,849 B1). High temperature distillation or rectification of carbon dioxide and ammonia at an elevated temperature has been proposed (U.S. Pat. Nos. 4,710,300 and 6,368,849 B1). Membrane processes with decarbonization and pH adjustment have likewise been proposed. Pressurizing the digester contents and driving CO2 into solution has also been practiced. All these processes require a significant investment in energy for heat and pressure, and reagents for pH adjustment. Scale formation is a common problem if calcium or magnesium is used to adjust pH. Rectification or high temperature stripping requires the removal of most suspended solids prior to high temperature steam stripping or rectification.
Following ammonia stripping the ammonia can be sequestered through a variety of means. If high-temperature distillation is used to remove both carbon dioxide and ammonia, the uncontrolled formation of ammonium bicarbonate solids (scale) can be mechanically removed from the stripping unit (U.S. Pat. No. 4,710,300). If the ammonia is stripped with air or steam, anhydrous ammonia or aqueous ammonia can be formed at a reduced pH (U.S. Pat. No. 6,464,875, U.S. Pat. No. 5,702,572). If ammonia is stripped with air or steam ammonium salts can be formed through a reaction with a dilute acid (U.S. Pat. No. 6,521,129).
Biological processes have been used to remove ammonia nitrogen. They include aerobic nitrification and denitrification and the anaerobic Anammox process whereby ammonia is anaerobically converted to nitrogen gas resulting in the loss of ammonia nitrogen's fertilizer value.
High temperature reduced pressure stripping, as well as distillation to remove both carbon dioxide and ammonia will improve the biogas quality since a portion of the carbon dioxide is removed from the gas stream under the high temperature conditions (U.S. Pat. No. 4,710,300). Improved gas quality has also been claimed when digesting a substrate having a high concentration of nitrogen through the formation of ammonium bicarbonate in solution (U.S. Pat. No. 7,160,456 B2). Also, biogas quality improvements have been claimed for processes that pass biogas through the digester liquid containing ammonia to form ammonium carbonate in the liquid slurry (U.S. Pat. No. 4,372,856, and U.S. Pat. No. 7,160,456).
A variety of processes are utilized to directly improve the BTU content of biogas. These processes involve the removal of carbon dioxide by high-pressure water scrubbing (U.S. Pat. No. 6,299,774), amine scrubbing, and membrane separation. Most of the systems involved high-pressure operation with significant capital and operation and maintenance costs. Biological processes have also been used such as acid phase anaerobic digestion (U.S. Pat. No. 5,529,692) where the CO2, formed in the acid phase, is separately removed from the predominately methane gas stream from the methane phase.
The economics of ammonia removal and sequestration, as well as the production of a high BTU biogas, can be improved significantly by operating a low pressure, low temperature, process that can remove substantially all of the ammonia while controlling the quality of the biogas produced. The process would be even more advantageous if it can be performed without the use of costly chemical reagents that increase the salt content of the effluent, or costly energy in the form of heat and steam.